1. Field of the Invention
The present invention relates to a pneumatic tire in which, at the tread of the pneumatic tire, land portions, which extend in the circumferential direction of the tire and are separated by a plurality of main grooves extending in the circumferential direction of the tire, are separated into a plurality of blocks by grooves extending in a direction intersecting the circumferential direction of the tire.
2. Description of the Related Art
Pneumatic tires have been known in which, at the tire tread, land portions, which extend in the circumferential direction of the tire and are separated by a plurality of main grooves extending in the circumferential direction of the tire, are divided by plural grooves extending in a direction intersecting the circumferential direction of the tire.
It has also been known that performance of a tire in driving on wet roads can be improved by forming sipes in the tread or by increasing the negative ratio (the fraction of depressed portions) in the tread.
It has also been known that rolling resistance of a tire can be reduced by enhancing rigidity of land portions of the tread by not forming sipes or other methods, so as to suppress deformation.
However, when sipes are formed to an excessive degree to improve performance in driving on wet roads, a problem arises in that, although the edge component increases, the rigidity of the blocks decreases which causes a deterioration in the road-contacting property of the blocks and sufficient force is not generated.
When sipes are formed to an excessive degree, another problem arises in that deformation of the blocks increases and thus rolling resistance increases. Therefore, in conventional tires, it has not been possible to simultaneously achieve improved performance in driving on wet roads and reduced rolling resistance.
The present invention has been made to overcome the above problems, and an object of the present invention is to provide a pneumatic tire which can exhibit improved performance in driving on wet roads and reduced rolling resistance simultaneously.
In the pneumatic tire of the first aspect of the present invention, a tread comprises a plurality of main grooves extending in a circumferential direction of the tire and a plurality of land portions separated by the plurality of main grooves, said land portion comprising a row of a plurality of blocks divided in the circumferential direction of the tire by a plurality of transverse grooves which are open to main grooves at both sides in an axial direction of the tire, wherein the block has a closed sipe which is disposed at an approximately central portion of the block and whose both longitudinal direction end portions are located inside the block, the transverse grooves are disposed so as to be inclined in one direction with respect to the axial direction of the tire in one land portion, and the closed sipe is disposed so as to be inclined in a direction opposite to the direction of the transverse grooves with respect to the axial direction of the tire or is disposed parallel to the axial direction of the tire.
The pneumatic tire of the first aspect of the present invention exhibits improved performance in driving on wet roads at the time of driving and braking because the transverse grooves and the closed sipes are formed in the land portions. Moreover, because the transverse grooves and the closed sipes are inclined with respect to the axial direction of the tire, the performance in driving on wet roads at the time of making turns is improved by the effect of the tire circumferential direction edge components of the transverse grooves and the closed sipes.
When a block contacts the road surface, the pressure from the road surface tends to concentrate at the central portion of the block. However, the pneumatic tire of the first aspect has a closed sipe at an approximately central portion of the block. Therefore, the pressure from the road surface can be dispersed to both sides of the closed sipe and the high pressure from the road surface at the central portion of the block can be reduced.
In general, it is inevitable that formation of a sipe in a block causes a decrease in the rigidity of the block. However, when the sipe is formed in a manner such that the sipe is a closed sipe having both end portions located inside the block and is disposed parallel to the axial direction of the tire or inclined in the direction opposite to the direction of the transverse grooves with respect to the axial direction of the tire, the geometrical moment of inertia of the block can be maintained and sufficient rigidity of the block can be maintained, or, in other words, an excessive decrease in the rigidity of the block can be prevented.
Therefore, in the pneumatic tire of the first aspect, deformation of the blocks due to contact with the road surface can be suppressed, and rolling resistance can be reduced.
When a so-called open sipe having both end portions open to the sides of a block is formed in the block, the block is completely divided into portions and the rigidity of the block decreases to a great extent. Therefore, deformation of the block due to contact with the road surface increases and rolling resistance increases.
Even when a closed sipe is formed in a block, if the closed sipe is disposed in the same direction as that of the transverse grooves dividing the respective blocks, the rigidity of the block decreases, deformation of the block due to contact with the road surface increases, and rolling resistance increases.
In the pneumatic tire of the second aspect of the present invention in the pneumatic tire of the first aspect of the present invention, an angle between the transverse groove and the axial direction of the tire is 60xc2x0 or less and an angle between the closed sipe and the axial direction of the tire is 60xc2x0 or less.
When the angle between the transverse groove and the axial direction of the tire exceeds 60xc2x0, corner portions of the block have an excessively sharp angle which causes a decrease in the rigidity of the block. Deformation of the block due to contact with the road surface increases and rolling resistance increases, which is not preferable. The block also tends to deform in the axial direction of the tire due to the side force. Therefore, such angles are not preferable.
When the angle between the closed sipe and the axial direction of the tire exceeds 60xc2x0, even if the length in the longitudinal direction of the sipe is increased, the edge components in the axial direction of the tire which are required to obtain satisfactory performance in driving on wet roads at the time of braking and driving decrease (to one half or less). Therefore, such angles are not preferable.
Therefore, it is preferable that the angle between the transverse groove and the axial direction of the tire is 60xc2x0 or less and the angle between the closed sipe and the axial direction of the tire is 60xc2x0 or less, in order to simultaneously obtain sufficient rigidity of the block and improved performances in driving on wet roads at the times of making turns, driving and braking.
In the pneumatic tire of the third aspect of the present invention in the pneumatic tires of either of the first aspect or the second aspect of the present invention, given that the dimension of the closed sipe along the axial direction of the tire is represented by Wb and the dimension of the land portion along the axial direction of the tire is represented by Wa, Wb and Wa satisfy the relation Wbxe2x89xa6⅔Wa.
When the dimension Wb of the closed sipe along the axial direction of the tire exceeds ⅔ of the dimension Wa of the land portion along the axial direction of the tire, rigidity of the block excessively decreases, deformation of the block at the time of contact with the road surface increases, and rolling resistance increases. Therefore, such a condition is not preferable.
Accordingly, it is preferable that the relation Wbxe2x89xa6⅔Wa is satisfied to maintain sufficient rigidity of the block.
In the pneumatic tire of the fourth aspect of the present invention in any of the pneumatic tires of the first aspect, the second aspect and the third aspect of the present invention, given that a depth of the closed sipe is represented by h and a depth of the main groove is represented by H, h and H satisfy the relation ({fraction (3/10)})Hxe2x89xa6hxe2x89xa6({fraction (8/10)})H.
When the depth h of the closed sipe is smaller than {fraction (3/10)} of the depth H of the main groove, the ability to reduce the high pressure from the road surface at the central portion of the block deteriorates. Therefore, such a condition is not preferable.
When the depth h of the closed sipe exceeds {fraction (8/10)} of the depth H of the main groove, the rigidity of the block excessively decreases. Therefore, deformation of the block at the time of contact with the road surface increases, and rolling resistance increases. Therefore, such a condition is not preferable.
Therefore, it is preferable that the relation ({fraction (3/10)})Hxe2x89xa6hxe2x89xa6({fraction (8/10)})H is satisfied to maintain sufficient rigidity of the block while the high pressure from the road surface at the central portion of the block is reliably reduced.
In the pneumatic tire of the fifth aspect of the present invention in any of the pneumatic tires of the first aspect, the second aspect, the third aspect and the fourth aspect of the present invention, a width of the closed sipe is 2.0 mm or less.
An excessive decrease in the rigidity of the block can be prevented by keeping the width of the closed sipe at 2.0 mm or less.
When the width of the closed sipe exceeds 2.0 mm, the rigidity of the block excessively decreases, deformation of the block at the time of contact with the road surface increases, and rolling resistance increases. Therefore, such a condition is not preferable.
In the pneumatic tire of the sixth aspect of the present invention in any of the pneumatic tires of the first aspect, the second aspect, the third aspect, the fourth aspect and the fifth aspect of the present invention, the closed sipe is formed along a shorter diagonal line of diagonal lines of the block.
By forming the closed sipe along the shorter diagonal line of the diagonal lines of the block, the high rigidity of the block can be maintained.